Prior art chip-scale ultra-violet lasers include diode pumped solid-state lasers, gas lasers, and fiber lasers. While chip-scale refers to the size of the devices, these prior art chip-scale ultra-violet lasers are relatively bulky in nature, and have low wall-plug efficiencies (WPEs) of less than 2%. Ultra violet lasing structures based on second harmonic generation (SHG) outside the cavity have been demonstrated. However, the demonstrated output power levels are extremely low (<1 μW) due to the inefficiency of the extra-cavity frequency doubling approach used, and the free-space implementation results in a lack of robustness for practical applications.
The shortest UV wavelength laser diode that has been demonstrated in the prior art has a wavelength of 336 nm, and is based on an Al0.06Ga0.94N/Al0.16Ga0.84N multiple quantum well (MQW) structure grown on a sapphire substrate. The WPE of this laser diode is an extremely low 0.014%, with an output power of about 3 mW in the pulse mode. The main reason for the poor performance of these laser diodes is the poor AlGaN material quality, which is in particular poor as the Al mole fraction is increased for shorter emission wavelengths. As an example, an AlGaN/GaN MQW laser diode emitting at 345 nm, again grown on a sapphire substrate, had a slightly improved WPE of about 0.077%, and a peak pulsed power of 35 mV as described in Reference 6 below, which is incorporated herein by reference.
Other research in this area includes optically pumped AlxGa1-xN heterostructure lasers grown on AlN substrates with emissions at 248 nm using a KrF excimer laser as a pump source. A threshold optical power density of 40 kW/cm2 has been demonstrated with this laser structure. The disadvantage of this approach is that the UV laser is optically rather than electrically pumped, making it less suitable for system insertions.
Frequency multiplication has also been used to achieve lasing in the UV band. For example, an average output power of 600 mW has been demonstrated using a two-stage extra-cavity frequency doubling of a Nd:YAG fiber laser emitting at 946 nm. The first stage (946-to-473 nm) using a BiBO (Bismuth Borate, BiB3O6) crystal had a conversion efficiency of 38%, while the second stage (473-236.5 nm) using a BBO (beta barium borate) crystal had a 17% conversion efficiency as described by Reference 7 below, which is incorporated herein by reference. Also, frequency quadrupling of a Ti-sapphire laser at 820 nm with a ps pulse train using LBO (Lithium triborate (LiB3O5)) and BBO crystals in resonant doubling cavities has been demonstrated to have a 25 mW output at 205 nm with an overall conversion efficiency of 4.5%.
Although no intra-cavity frequency doubling has been reported in the deep UV band with a wavelength of less than 240 nm, continuous wave (CW) intra-cavity generation at 320 nm UV wavelength with a conversion efficiency of 35% has been demonstrated using an optically pumped red-emitting Pr:BaY2F8 crystal and a LBO nonlinear optical crystal (NLC) as described in Reference 8 below, which is incorporated herein by reference. Also, over 5 W of continuous wave (CW) power in the visible (585 nm) has been demonstrated with intracavity frequency doubling using an optically pumped vertical extended cavity surface emitting laser (VECSEL) emitting at 1170 nm and a LBO NLC with a conversion efficiency of >58% as described in Reference 2 below, which is incorporated herein by reference.
Another possible chip-scale approach is to use a semiconductor-based nonlinear element, such as GaN, in a resonant cavity, such as a micro-ring resonator, to achieve on-chip frequency doubling. However, the second harmonic generation (SHG) power levels demonstrated so far using this approach have been quite low, with power levels on the order of a few microwatts measured at a wavelength of 780 nm using >100 mW of pump power at 1550 nm, which is a conversion efficiency of <0.01%. Furthermore, GaN cannot be used for second harmonic generation (SHG) generation at 220-240 nm wavelengths because of its absorption.